I enjoy aspects of all 4 seasons, and love that here in CO, we actually get all 4, unlike many places I've lived.
Spring is great for the promise of new life, temps rising into the reasonable range more frequently, and all the great scents. Seeing all the baby animals is great, as is the ability to once again cycle in shorts. Spring rules.
Summer is great all around. Hotter, but not enough to keep me out of the saddle. Long days can equal long rides. Heat chases many users away from the local MUPs, leading to better rides for those who are properly-hydrated and unafraid of higher temps. Summer rules.
Fall brings relief from summer's heat with the promise of winter. Here in CO, it also brings the fall color as the leaves of aspens, cottonwoods and other deciduous trees change color in a series of magnificent displays I will someday get to enjoy from my saddle: still working on getting into the mountains proper for some serious roadie work. Can't wait to make it happen. Fall rules.
Winter brings cold and lots of it, but that's what protective gear is for. The local MUPs largely clear out when temps drop into the sub-50s, even though they are plowed every time it snows significantly. I love the way snow utterly changes a landscape. I love the looks I get from the few other users out, almost all of whom are heavily-bundled pedestrians, when I roll by. The feeling of companionship I feel with the few other winter cyclists I see is just awesome. It's good to be crazy, sometimes. Winter rules.
Ultimately, I love seeing the progression from one season to the next. It's wonderful to get/be out experiencing the weather day to day rather than huddling inside in the heat/AC of one's house/car. Every season brings with it positive and negative aspects. I try to focus on the benefits gained and detrments shed with each change of season. They'll roll around whether we like it or not; may as well focus on the benefits of a change which is totally beyond our ability to control.